Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon image-wise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
Alternatively, printing plates are known that include a photosensitive coating that upon image-wise exposure is rendered soluble at the exposed areas. Subsequent development then removes the exposed areas. A typical example of such photosensitive coating is a quinone-diazide based coating.
Typically, the above described photographic materials from which the printing plates are made are camera-exposed through a photographic film that contains the image that is to be reproduced in a lithographic printing process. Such method of working is cumbersome and labor intensive. However, on the other hand, the printing plates thus obtained are of superior lithographic quality.
Attempts have thus been made to eliminate the need for a photographic film in the above process and in particular to obtain a printing plate directly from computer data representing the image to be reproduced. In particular it has been proposed to coat a silver halide layer on top of the photosensitive coating. The silver halide can then directly be exposed by means of a laser under the control of a computer. Subsequently, the silver halide layer is developed leaving a silver image on top of the photosensitive coating. That silver image then serves as a mask in an overall exposure of the photosensitive coating. After the overall exposure the silver image is removed and the photosensitive coating is developed. Such method is disclosed in for example JP-A-60/61752 but has the disadvantage that a complex development and associated developing liquids are needed.
GB-A-1 492 070 discloses a method wherein a metal layer or a layer containing carbon black is provided on a photosensitive coating. This metal layer is then ablated by means of a laser so that an image mask on the photosensitive layer is obtained. The photosensitive layer is then overall exposed by UV-light through the image mask. After removal of the image mask, the photosensitive layer is developed to obtain a printing plate. This method however still has the disadvantage that the image mask has to be removed prior to development of the photosensitive layer by a cumbersome processing.
Systems are also known where a mask is image-wise formed on a photosensitive layer by transfer of a masking substance to the photosensitive coating, e.g. by means of laser transfer or xerography as disclosed in EP-A-1 138. However, such method is generally slow and may not meet the required image resolutions for obtaining high quality prints.
EP-A-803 770 discloses an imaging element for making a lithographic printing plate wherein that imaging element comprises a thermosensitive mask on a photosensitive coating, said thermosensitive mask comprising a masking dye which is capable of being ablated upon exposure with laser light. EP-A-803 769, EP-A-803 771 and EP-A-803 772 describes imaging elements of an analogous kind. A problem that arises with the present ablation based printing plate precursors is that said ablation process can cause formation of debris originating from the ablatable layer itself or of other functional layers of said precursor. Said debris can interfere with transmission of the laser beam (e.g. by depositing on a focusing lens or as an aerosol that partially blocks transmission) or with the transport of the imaging element during or after recording when this debris remains loosely adhered to the plate and deposition of said debris occurs on the transport rollers